The physical downlink shared channel (PDSCH) and the physical uplink shared channel (PUSCH) in Long Term Evolution (LTE) systems are time and frequency multiplexed channels shared by a plurality of user terminals. User terminals periodically send channel quality indication (CQI) reports to a base station. The CQI reports indicate the instantaneous channel conditions as seen by the receivers in the user terminals. During each one millisecond subframe interval, commonly referred to as a transmission time interval (TTI), a scheduler at the base station schedules one or more user terminals to receive data on the PDSCH or to transmit data on the PUSCH, and determines the transmission format for the downlink/uplink transmissions. The identity of the user terminals scheduled to receive/transmit data in a given time interval and the transmission format, are transmitted to the user terminals on the physical downlink control channel (PDCCH) in a traffic channel assignment message.
In many circumstances, multiple downlink transmissions may be scheduled in a single TTI. For example, in Voice Over IP (VOIP) applications, it is desirable to schedule multiple VOIP users in the same TTI in order to increase the number of VIIP users that can be supported by the cell. Also, in data applications, a single user may have a limited amount of data to send in a TTI. If only a single user is scheduled during a TTI, there will be many instances where the resources are not fully used. Allowing multiple users to receive/transmit data during the same TTI avoids the unnecessary wasting of radio resources.
Allowing multiple users to be scheduled in the same TTI enables the base station to take advantage of multi-user diversity to increase system capacity. In the case of a frequency selected channel, the channel conditions as seen by the receivers will vary in the frequency domain. A first set of sub-bands may be best for a first user, while a second set of sub-bands is best for a second user. System capacity is increased by assigning, to each user, the sub-bands that are the best for that user.
When multiple users are scheduled in the same TTI, the base station needs to send multiple assignment messages. As noted above, the assignment messages are transmitted on the PDCCH. In LTE, a certain number of orthogonal frequency division multiplexing (OFDM) symbols is reserved for the PDCCH. The PDCCH comprises a plurality of control channel elements (CCEs), which are consecutively numbered. When a user is scheduled, the base station selects a CCE set from a group of candidate sets to send the assignment message to the user terminal. The number of CCEs in the set, referred to as the aggregation level, may vary, depending on the channel conditions and the amount of information that needs to be transmitted. Thus, a CCE set may comprise one, two, four, or eight CCEs. For a given user terminal at certain TTI, the locations or the indexes of the CCEs belonging to the group of candidate sets are specified by 3GPP standards. Some CCEs in the group of candidate sets for one user terminal may overlap with CCEs in the group of candidate sets for another user terminal.
When allocating CCEs to the PDCCHs of the user terminals, the typical approach is to assign CCEs to the user terminals in order of scheduling priority, beginning with the user terminal having the highest scheduling priority. This priority based scheduling approach can lead to low PDCCH capacity. During the allocation process, the available CCEs for the PDCCHs are consecutively numbered. The CCEs are allocated in blocks, depending on the aggregation level for the user terminals. For example, a user terminal with an aggregation level of eight will be assigned a CCE set comprising eight contiguous CCEs. Because the PDCCHs of the user terminals are constrained to start on certain CCEs, a user terminal can be blocked if every possible CCE candidate set contains at least one CCE that has been previously allocated to another user terminal. Blocking of a PDCCH can occur even though there is a sufficient number of available CCEs.
Another problem with the priority based allocation is priority inversion. Due to blocking situations, resources may be allocated to the PDCCH for a lower priority user while a higher priority user is blocked. Priority inversion may occur when the higher priority user requires a higher aggregation level than the lower priority user.
The priority based allocation approach may also result in unnecessary intercell interference. In situations where less than all of the available CCEs are needed for the PDCCHs, intercell interference can be reduced in synchronized networks by aligning the unused CCEs in one cell with the used CCEs in an adjacent cell. The priority based resource allocation approach does not consider alternative allocations that might reduce interference with adjacent cells.
A simple way to increase PDCCH capacity is to reserve more OFDM symbols for the PDCCH. More OFDM symbols, however, would increase the signaling overhead and reduce the number of OFDM symbols available for transmission of data.